The present invention relates to a magnetic sensor arrangement, in particular for sensing the movement of elements moved in linear or rotary fashion.
It is intrinsically known that magnetic field-sensitive sensors are used in many areas of use in which a contact-free detection of a movement is desired. This motion can be rotary or linear. It is necessary to draw a distinction here between two different basic measurement principles. On the one hand, attaching one or more active elements in the form of magnetic dipoles to the element to be detected makes it possible to determine the movement directly by means of the chronologically changing magnetic field at the sensor location. By contrast, with passive transmitter elements, which are comprised of a soft magnetic material, the magnetic field is generated by a working magnet that is affixed to the sensor. The sensor measures the change in the magnetic field of the working magnet induced by the movement of the transmitter elements.
In addition to the known Hall technology for magnetic field measurement, even with passive transmitter elements, the alternative use of so-called XMR technologies, i.e. magnetoresistive measurement principles, is on the rise in the automotive field. It should be noted that by contrast with Hall sensors, XMR sensors detect the so-called “in-plane” component of the magnetic field in the sensor element. Prior conventional XMR sensors use a working magnet for this purpose, whose field must be balanced so that the offset at the location of the sensitive element is zero or so else a so-called back-bias field is generated, which defines the operating point of the sensor.
For example, DE 101 28 135 A1 has described an approach in which a hard magnetic layer is deposited in close proximity to a magnetoresistive lamination stack, i.e. especially on top of and/or underneath it. This hard magnetic layer then primarily couples via its stray field into the magnetoresistive layers, thus generating a so-called bias magnetic field that functions as a magnetic field offset so that even with an only slight variation of an external magnetic field overlapping the internal magnetic field, it is possible to achieve an easily measurable, relatively large change in the actual measurement value, which is detected as a resistance change in the layer arrangement.
The above-described sensors are used in an intrinsically known fashion for detecting rotational speed, for example in the automotive field, often embodied in the form of a so-called gradiometer arrangement. This means that each pair of branches of a Wheatstone measuring bridge are spaced a predetermined distance apart so that a homogeneous magnetic field does not generate a bridge signal. A variation of the magnetic field in the region of the predetermined distance does, however, generate a bridge signal. As a result, the sensor measures only the signal of a magnetic claw-pole rotor whose pole pair distance corresponds approximately to the predetermined gradiometer distance.
In contrast with the absolutely measuring XMR elements, applying the gradiometer principle in a magnetoresistive XMR measuring bridge makes it possible to achieve a reduction in the sensitivity of the sensors to homogeneous interference fields. But it is no longer possible in this case to carry out a balancing of the previously used magnet so that the offset can be eliminated at both locations of the sensor elements of the gradiometer arrangement; an electronic balancing is in principle possible, but in this case, a large offset yields a relatively weak signal.